Thin film transistor and method for manufacturing same

ABSTRACT

A thin film transistor including a gats electrode, a gate-insulating film, an oxide semiconductor film in contact with the gate-insulating film, and source and drain electrodes which connect to the oxide semiconductor film and are separated with a channel part therebetween, wherein the oxide semiconductor film comprises a crystalline indium oxide which includes hydrogen element, and the content of the hydrogen element contained in the oxide semiconductor film is 0.1 at % to 5 at % relative to all elements which form the oxide semiconductor film.

TECHNICAL FIELD

The invention relates to a thin film transistor having a crystallinesemiconductor film comprising indium oxide which contains a hydrogenelement, and a method for manufacturing the same.

BACKGROUND ART

In recent years, remarkable progress has been attained in displays.Various displays such as liquid crystal displays and EL displays havebeen actively incorporated in an OA apparatus such as a PC and a wordprocessor. Each of these displays has a sandwich structure in which adisplay element is disposed between transparent conductive films.

Currently, a silicon-based semiconductor film has been used mainly as aswitching device of a thin film transistor (TFT) or the like which isused to drive the above-mentioned display. The reason therefor is that,in addition to improved stability and processibility of a silicon-basedthin film, a thin film transistor using a silicon-based thin film hasadvantages such as a high switching speed. Generally, this silicon-basedthin film is fabricated by the chemical vapor deposition (CVD) method.

However, in the case of an amorphous silicon-based thin film, there aredisadvantages that the switching speed is relatively low and imagescannot be displayed when a high-speed animation or the like aredisplayed, Further, in the case of a crystalline silicon-based thinfilm, although the switching speed is relatively high, heating at a hightemperature of 800° C. or higher, heating by means of a laser or thelike is required, and hence, a large amount of energy and a large numberof steps are required in production. Although a silicon-based thin filmexhibits superior performance as a voltage element, it encounters aproblem that its properties change with the passage of time when currentis flown.

An oxide semiconductor has attracted attention as a material or the likewhich is used to obtain a transparent semiconductor film which issuperior to a silicon-based thin film in stability and has lighttransmittance equivalent to that of an ITO film.

However, in a film containing crystals of indium oxide, in particular apolycrystalline film, oxygen deficiency tends to occur easily. It isbelieved that it is impossible to allow the carrier density to be2×10⁺¹⁷ cm⁻³ or less even if the oxygen partial pressure during filmformation is increased, an oxidization treatment or the like isconducted. Therefore, no attempt has been made to use this film as asemiconductor film or as a TFT.

Under such circumstances, Patent Document 1 discloses a thin filmtransistor having a semiconductor layer comprising indium oxide.Specifically, this document discloses a method in which a thin filmtransistor is obtained by subjecting an indium oxide film to a heattreatment under an oxidizing atmosphere. However, in the case of a thinfilm formed of indium oxide, performance of the resulting thin filmtransistor varies depending on heat treatment conditions, oxidizingatmosphere conditions, in particular, on humidity conditions when a heattreatment is conducted in the air, resulting in unstable transistorperformance.

On the other hand, Patent Documents 2 and 3 disclose that an amorphousoxide semiconductor can be stably obtained due to the presence of ahydrogen element or a deuterium element in an amorphous oxidesemiconductor film. However, since an amorphous oxide semiconductor filmhas problems that, due to its amorphous nature, a hydrogen element or adeuterium element within the film may be diffused in the air, or watermolecules may enter the film from the air, allowing the amount of ahydrogen element in the film to be excessive and hence, making theresulting device to be unstable.

If crystalline indium oxide is used in a semiconductor film, since theresulting semiconductor film is not dissolved in oxalic acid, PAN or thelike, which means that the film has etching resistance, there areadvantages that a channel-etch TFT can be produced easily. However, itis significantly difficult to allow a crystalline indium oxide filmalone to be semiconductive by sufficiently lowering the carrier densitythereof, That is, if an indium oxide film is merely crystallized,carriers are generated due to oxygen deficiency or the presence of apositive tetravalent metal oxide which is a coexistent impurity, wherebythe oxide indium film may become a conductor. Therefore, a TFT whichuses crystalline indium oxide in a semiconductor film has not beenfabricated so far.

RELATED ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-2008-130814-   Patent Document 2: JP-A-2007-73697-   Patent Document 3: JP-A-2007-10391S

SUMMARY OF THE INVENTION

The object of the invention is to provide a thin film transistor whichhas stable performance even if heat treatment conditions duringproduction, in particular, humidity conditions or the like when a heattreatment is conducted in the air, vary.

The inventors made intensive studies to attain the above-mentionedobject. As a result, they have found that a high-performance thin filmtransistor can be obtained by using indium oxide containing apredetermined amount of a hydrogen element in a semiconductor film, andthat, in the formation of a semiconductor film, a desired semiconductorfilm can be stably obtained by forming an amorphous indium oxide filmcontaining a hydrogen element, and subsequently by subjecting the oxidefilm to a dehydrogenation treatment, thereby to control the amount ofhydrogen contained therein. The invention has been made based on such afinding.

According to the invention, the following thin film transistor or thelike are provided.

1. A thin film transistor comprising a gate electrode, a gate-insulatingfilm, an oxide semiconductor film in. contact with the gate-insulatingfilm, and source and drain electrodes which connect to the oxidesemiconductor film and are separated with a channel part therebetween,

wherein the oxide semiconductor film comprises a crystalline indiumoxide which comprises a hydrogen element, and

the content of the hydrogen element contained in the oxide semiconductorfilm is 0.1 at % to 5 at % relative to all elements which form the oxidesemiconductor film.

2. The thin film transistor according to 1 wherein the oxidesemiconductor film further comprises a positive trivalent metal oxideother than indium oxide,3. The thin film transistor according to 2 wherein the content of thepositive trivalent metal oxide other than indium oxide is 0.1 to 10 at %relative to all elements contained in the oxide semiconductor film,4. The thin film transistor according to 2 or 3 wherein the positivetrivalent metal oxide other than indium oxide is one or more oxidesselected from boron oxide, aluminum oxide, gallium oxide, scandiumoxide, yttrium oxide, lanthanum oxide, praseodymium oxide, neodymiumoxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide,dysprosium oxide, holmium oxide, erbium oxide, ytterbium oxide, andlutetium oxide.5. A method for producing the thin film transistor according to any oneof 1 to 4 comprising the steps of,

-   -   forming a semiconductor film comprising indium oxide which        comprises a hydrogen element,    -   patterning the semiconductor film,    -   dehydrogenating and crystallizing the semiconductor film, and    -   forming source and drain electrodes such that the electrodes        connect to the semiconductor film.        6. The method for producing a thin film transistor according to        5 wherein in the step of forming the semiconductor film, the        content in volume of hydrogen molecules and/or water molecules        in the film-forming atmosphere is 1% to 10%.        7. The method for producing a thin film transistor according to        5 or 8 wherein in the step of dehydrogenating and crystallizing        the semiconductor film, the semiconductor film is subjected to a        heat treatment at 150 to 450° C. for 0.1 to 1200 minutes.        8. The method for producing a thin film transistor according to        any one of 5 to 7 which is a method for producing a channel etch        thin film transistor.        9. The method for producing a thin film transistor according to        any one of 5 to 7 which is a method for producing an        etch-stopper thin film transistor.

According to the invention, a high-performance thin film transistor canbe stably obtained even though heat-treatment conditions duringproduction vary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an embodiment of achannel-etch thin film transistor of the invention;

FIG. 2 is a schematic cross-sectional view showing an embodiment of anetch-stopper thin film transistor of the invention;

FIG. 3 is a schematic cross-sectional view of a channel-etch thin filmtransistor fabricated in Example 1; and

FIG. 4 is a schematic cross-sectional view of an etch-stopper thin filmtransistor fabricated in Example 2.

MODE FOR CARRYING OUT THE INVENTION

The thin film transistor (TFT) of the invention comprises a gateelectrode, a gate-insulating film, an oxide semiconductor film incontact with the gate-insulating film, and source and drain electrodeswhich connect to the oxide semiconductor film and are separated with achannel part therebetween. This thin film transistor is characterized inthat the oxide semiconductor film comprises a crystalline indium oxidesemiconductor film which comprises a hydrogen element.

FIG. 1 is a schematic cross-sectional view showing an embodiment of thethin film transistor of the invention.

A thin film transistor 1 has a gate electrode 20 between a substrate 10and an insulating film 30. On the gate insulating film 30, asemiconductor film 40 is stacked as an active layer. Further, a sourceelectrode 50 and a drain electrode 52 are formed such that theyrespectively cover near the end parts of the semiconductor film 40. Achannel part 60 is formed in an area surrounded by the semiconductorfilm 40, the source electrode 50 and the drain electrode 52.

The thin film transistor 1 shown in FIG. 1 is a so-called channel-etchthin film transistor. The thin film transistor of the invention is notlimited to a channel-etch thin film transistor, and a deviceconfiguration which is known in this technical field can be used.

FIG. 2 is a schematic cross-sectional view showing another embodiment ofthe thin film transistor of the invention. The constituting elementswhich are the same as the constituting elements of the above-mentionedthin film transistor 1 are indicated by the same numerals, and anexplanation thereof is omitted.

A thin film transistor 2 is an etch-stopper Shin film transistor. Thethin film transistor 2 has the same constitution as that of theabove-mentioned thin film transistor 1 except that an etch stopper 70 isformed so as to cover the channel part 60. The source electrode 50 andthe drain electrode 52 are formed such that they respectively cover nearthe end parts of the semiconductor film 40 and near the end parts of theetch stopper 70.

In the invention, as the semiconductor film 40, a crystalline indiumoxide semiconductor film comprising indium oxide which contains ahydrogen element is used. Since the crystalline indium oxide filmcontains a hydrogen element, performance of the thin film transistor isstabilized. Indium oxide is a compound which tends to cause oxygendeficiency, and hence, it is used as a material for a transparentconductive film, it is believed that, since a hydrogen element fills adeficiency generated by oxygen deficiency to suppress generation ofcarriers, stabilization of a semiconductor is realized.

Further, due to the presence of a hydrogen element, it is possible todecrease the carrier concentration of the semiconductor film,specifically, to less than 2×10⁺¹⁷ cm⁻³, at a temperature around roomtemperature, whereby good thin film transistor properties are exhibited.

Meanwhile, it is preferred that the carrier density of the semiconductorfilm at a temperature around room temperature be less than 2×10⁺¹⁷ cm⁻³.If the carrier density is 2×10⁺¹⁷ cm⁻³ or more, the resulting transistormay not be driven as a TFT. Even if it is driven as a TFT. the TFT mayoperate in the normally-on state, have a significantly negativethreshold voltage or have a small on-off value.

The content of a hydrogen element in the semiconductor film ispreferably 0.1 to 5 at %, particularly preferably 0.5 to 3 at % relativeto ail elements contained in the semiconductor film. If the content isless than 0.1 at %, since the content is too small, an indium oxide thinfilm tends to become a conductive film, and stable TFT properties maynot be obtained.

If the content of a hydrogen element exceeds 5 at %, a thin film may bean insulator film.

In the semiconductor film, a hydrogen element may be present in the formof a molecule or in the form of an atom. Further, it may connect tooxygen to be present in the form of a hydroxyl group, it is preferredthat a hydrogen element present in the form of a hydroxyl group.

The content of hydrogen can be measured by the Rutherford backscatteringspectrometry (RBS) method, the hydrogen forward scattering spectrometry(HFS) method or the thermal deposition spectrometry (TDS) method. Inthis invention, the content of a hydrogen element means a value measuredby the hydrogen forward scattering spectrometry (HFS) method.

The content of a hydrogen element in the semiconductor film can becontrolled by adjusting the hydrogen concentration in a film-formingatmosphere of the semiconductor film or the temperature or time of adehydrogenation process after the film formation.

In the invention, a crystalline semiconductor film is used. By using acrystalline semiconductor film, mobility of a TFT can be increased. Atthe same time, durability can also be enhanced. In addition, it ispossible to prevent the semiconductor film from being etched duringetching of the source electrode 50 and the drain electrode 52.

Here, the “crystalline film” is a film of which the crystal peak can beconfirmed by the X-ray diffraction analysis. Although a crystalline filmmay be a monocrystal film, an epitaxial film or a polycrystalline film,it is preferably an epitaxial film or a polycrystalline film since itcan easily enables industrial production and an increase in size. Apolycrystalline film is particularly preferable.

When a crystalline film is a polycrystalline film, it is preferred thatthe polycrystalline film be formed of nanocrystals. The average crystalparticle size obtained by using Scherrer's equation from the X-raydiffraction analysis is normally 500 nm or less, preferably 300 nm orless, more preferably 150 nm or less and further preferably 80 nm orless, if the average crystal particle size exceeds 500 nm, properties ofa transistor may vary largely if a transistor is miniaturized.

In the invention, it is further preferred that the semiconductor filmcontain a positive trivalent metal oxide other than indium oxide. As aresult, oxygen deficiency generated in crystalline indium oxide can besuppressed easily, and hence, it is possible to obtain a thin filmtransistor which is operated stably.

As a positive trivalent metal oxide other than indium oxide, boronoxide, aluminum oxide, gallium oxide, scandium oxide, yttrium oxide,lanthanum oxide, praseodymium oxide, neodymium oxide, samarium oxide,europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide,holmium oxide, erbium oxide, ytterbium oxide or lutetium oxide canpreferably be used. These oxides may be used singly or in combination oftwo or more.

In respect of the fact that indium oxide including a positive trivalentmetal oxide (excluding indium oxide) is crystallized more easily, it ispreferred that the ionic radius of the metal element of a positivetrivalent metal oxide to be added be as closer as possible to the ionicradius of the indium element. Specifically, one having an ionic radiuswhich is different from that of the indium element within a range of±30% is more preferably used. If the difference in ionic radius fromthat of the indium element exceeds 30%, the solubility limit may bedecreased or the solid solution may not be occurred. In such a case, apositive trivalent metal may be solid soluble interstitially betweencrystal lattices, or a positive trivalent metal may be segregated in thecrystal boundary. If the positive trivalent metal is segregated in thecrystal boundary, it has an effect of suppressing oxygen deficiencyoccurred in the crystal boundary.

From the above-mentioned point of view, as the positive trivalent metaloxide (excluding indium oxide), gallium oxide, scandium oxide, yttriumoxide, neodymium oxide, samarium oxide, europium oxide, gadoliniumoxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide andytterbium oxide are preferable, in particular.

The content of the positive trivalent metal oxide contained in thesemiconductor film (excluding indium oxide) is preferably 0.1 to 10 at%, in particular 0.5 to 8 at %, as a metal element amount relative toall metal elements contained in the semiconductor film. If the contentof the positive trivalent metal excluding indium is less than 0.1 at %,the amount of a positive trivalent metal oxide excluding indium oxide tobe added is small, and effects thereof may not be sufficient enough toobtain a normally-off thin film transistor. When the content of thepositive trivalent metal element exceeds 10 at %, the amount to be addedis too large, whereby a crystalline indium oxide film may not beobtained. If a semiconductor film is formed of amorphous oxide indium,the carrier concentration may not be decreased, and the thin filmtransistor may become normally on or the mobility of the resultingtransistor may not be improved.

The amount ratio of metal elements can be obtained by measuring theabundance of each element by the ICP-Mass (Inductively Coupled PlasmaMass) measurement.

In the invention, it is preferred that the content of a metal elementhaving an atomic valency of positive tetravalency or higher be 10 ppm(in the present application, the “ppm” means “atomic ppm”) or less. Ametal element having an atomic valency of positive tetravalency orhigher is present as an oxide in a semiconductor film. If a positivetetravalent metal oxide is captured in a crystal of indium oxide, itgenerates carriers within indium oxide to exert great effects on theperformance of the semiconductor film. Further, due to heat treatmentconditions of a semiconductor film, a metal element having an atomicvalency of positive tetravalency or higher is solid-solution substitutedin indium oxide to form an impurity level in the band structure ofindium oxide, thereby affecting semiconductor properties. As a result,the carrier density at a temperature around room temperature may not becontrolled to less than 2×10⁺¹⁷ cm⁻³. Therefore, a smaller content of ametal element with an atomic valency of positive tetravalency or higheris preferable. The content of a metal element with an atomic valency ofpositive tetravalency or higher is 5 ppm or less, with 1 ppm or lessbeing more preferable.

Examples of an oxide of a metal with an atomic valency of positivetetravalency or higher contained in the semiconductor film include anoxide of a heavy metal with an atomic valency of positive teteravalencyor higher such as titanium oxide, zirconium oxide, hafnium oxide,vanadium oxide, niobium oxide, tantalum oxide, chromium oxide,molybdenum oxide, tungsten oxide, manganese oxide, and silicon oxide,germanium oxide, tin oxide, lead oxide, antimony oxide, bismuth oxideand cerium oxide can be given.

Of the above-mentioned metal oxides, it is preferred that titaniumoxide, zirconium oxide and tin oxide, in particular, be strictlycontrolled.

In the invention, it is preferred that the content of a metal elementwith an atomic valency of positive divalency or lower relative to allmetal elements contained in the semiconductor film be 50 ppm or less. Ametal element with an atomic valency of positive divalency or lower ispresent as an oxide within the semiconductor film, if an oxide of ametal with an atomic valency of positive divalency or lower is capturedwithin a crystal of indium oxide, carrier traps may occur within indiumoxide. As a result, mobility may be lowered, exerting great effects onthe performance of the semiconductor film. Further, due to heattreatment conditions of a semiconductor film, a metal element having anatomic valency of positive divalency or lower is solid-solutionsubstituted in indium oxide to form an impurity level in the bandstructure of indium oxide, thereby affecting semiconductor properties.Therefore, a smaller content of a metal element with an atomic valencyof positive divalency or lower is preferable. The content of a metalelement with an atomic valency of positive divalency or lower is 10 ppmor less, with 5 ppm or less being more preferable.

Examples of an oxide of a metal with an atomic valency of positivedivalency or lower contained in the semiconductor film include alkalinemetal oxides or alkaline earth metal oxides such as lithium oxide,sodium oxide, potassium oxide, rubidium oxide, cesium oxide, magnesiumoxide, calcium oxide, strontium oxide and barium oxide or the like andzinc oxide.

Of the above-mentioned metal oxides, it is preferred that sodium oxide,potassium oxide, magnesium oxide, calcium oxide and zinc oxide bestrictly controlled.

In the thin film transistor of the invention, as for constitutingelements such as a substrate, a gate electrode, a gate insulating filmand a source/drain electrode, known elements can be used withoutparticular restrictions.

For example, a thin film of a metal such as Ai, Cu and Au can be usedfor each electrode, and a thin film of an oxide such as a silicon oxidefilm and a hafnium oxide film can be used for a gate insulating film.

Further, an insulating positive trivalent metal oxide film can be usedas an etch stopper. Preferred examples of the positive trivalent metaloxide include boron oxide, aluminum oxide, gallium oxide, scandiumoxide, yttrium oxide, lanthanum oxide, praseodymium oxide, neodymiumoxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide,dysprosium oxide, holmium oxide, erbium oxide, ytterbium oxide orlutetium oxide, for example. Silicon oxide, silicon nitride or the likemay be stacked on these films. Taking into consideration dry etchingproperties and cost, aluminum oxide, yttrium oxide or the like arepreferable.

In addition to a positive trivalent metal oxide, if silicon oxide or thelike are used in an etching stopper, for example, it may affect thesemiconductor film. Specifically, if a silicon oxide film is formed bysputtering, thermal CVD, plasma CVD or the like on amorphous indiumoxide which will serve as the semiconductor film, followed bycrystallization by heating, a silicon element may be diffused in theindium oxide film and solid-soluted. In such a case, since carriers maybe generated in the semiconductor film, causing the semiconductor filmto be conductive, the off current may be increased and the on/off ratiomay be decreased. Therefore, in a surface which is in contact with thesemiconductor film of an etch stopper, it is preferable to use aninsulating positive trivalent metal oxide film.

Next, a method for producing the thin film transistor of the inventionwill be explained.

The production method according to the invention comprises the steps offorming a semiconductor film comprising indium oxide which comprises ahydrogen element, patterning the semiconductor film, dehydrogenating andcrystallizing the semiconductor film, and forming source and drainelectrodes such that the electrodes connect to the semiconductor film.

Constituting elements such as a gate electrode, a gate insulating filmand source/drain electrodes can be formed by a known method.

For example, on a substrate, a gate electrode comprising a thin film ofa metal such as AI, Cu and Au is formed, and an oxide thin filmcomprising a silicon oxide film and hafnium oxide film or the like isformed as a gate insulating film. On the gate insulating film thusformed, by installing a metal mask, a semiconductor film comprising anindium oxide film is formed only on a necessary part. Thereafter, byusing a metal mask, source/drain electrodes are formed on a necessarypart, whereby a thin film transistor can be produced.

Hereinbelow, the film-forming step of the semiconductor film, which isthe characteristic part of the invention, will be explained.

The semiconductor film comprising indium oxide comprising a hydrogenelement can be formed by the sputtering method, the deposition method,the ion-plating method, the pulse laser deposition (PLD) method or thelike. Of these methods, the sputtering method is preferable.

As for the sputtering method, a method using a sintered target ispreferable, in particular, a sintered target comprising high-purity(having a purity of 99.99 at % or more, for example) indium oxide ispreferable. In order to form the above-mentioned semiconductor filmcomprising a positive trivalent metal oxide (excluding indium oxide), itsuffices to use a sintered target obtained by incorporating these metaloxides into indium oxide, for example. A sintered target can be producedby a method which is known in this technical field.

Sputtering conditions can be appropriately adjusted in accordance withthe kind of targets to be used, the thickness of the semiconductor filmor the like. As for the sputtering method, the RF sputtering method, theDC sputtering method and the AC sputtering method can be used. Of thesemethods, the DC sputtering method and the AC sputtering method arepreferable due to a high film-forming speed.

By introducing a hydrogen element in a film-forming atmosphere by theabove-mentioned method, an indium oxide semiconductor film containing ahydrogen element can be obtained. Specifically, film formation can beconducted in the state that hydrogen molecules (hydrogen gas) or wateris incorporated into a film-forming atmosphere.

The content in volume of hydrogen molecules and/or water molecules in afilm-forming atmosphere is preferably 1% to 10%, with 2% to 8% beingparticularly preferable.

As the method for allowing hydrogen molecules and/or water molecules tobe present in a film-forming atmosphere, a method in which an argon gascontaining hydrogen gas is used as a film-forming gas or a method inwhich water is directly sent to a film-forming chamber by means of aplunger pump or the like can be given. In the case of a gas, the contentin volume can be controlled by the partial pressure of each gascomponent.

In the invention, it is preferred that oxygen be present during filmformation of the semiconductor film. Due to the presence of oxygenduring sputtering, effective dehydrogenation can be conducted in adehydrogenation treatment step.

The semiconductor film thus obtained is then patterned. Patterning isconducted by a method such as wet etching and dry etching, if patternformation by means of a mask, pattern formation by means of lift-off orthe like is used in the formation of the semiconductor film, nopatterning is necessary. in the invention, it is preferred thatpatterning be conducted by wet etching or by means of a mask.

The semiconductor film is subjected to dehydrogenation andcrystallization treatments.

The dehydrogenation and crystallization steps have an effect ofcontrolling the amount of a hydrogen element, which has been excessivelyadded in indium oxide, to a fixed value. By conducting these steps, anoxide semiconductor film having stable performance can be alwaysobtained. Further, by conducting a dehydrogenation step (oxidizingstep), an indium oxide film is crystallized, whereby a thin filmtransistor having stable performance can be obtained.

As the step of subjecting the semiconductor film to a dehydrogenationstep or as the step of crystallizing the semiconductor film, a method inwhich hydrogen is oxidized with oxygen or a method in which hydrogenmolecules or water molecules are desorbed by heat can be mentioned.Specifically, a method such as heating in the air, heating in anon-oxidizing atmosphere (in an inactivated gas such as nitrogen orargon atmosphere), heating under vacuum can be used.

In the invention, a dehydrogenation treatment under vacuum or adehydrogenation treatment in a non-oxidizing atmosphere is preferable.

Here, the “under vacuum” means a state in which air is evacuated.Specifically, it means a state in which the pressure is 500 Pa or less,preferably 300 Pa or less, and more preferably 100 Pa or less. A methodof decreasing the degree of vacuum in the step-wise manner is alsopreferable.

As the heat treatment method, heating in an oven, contacting with aheating board (contact heating), lamp heating by means of an infraredlamp or the like, heating with light such as laser light, thermal plasmaheating or the like can be used.

It is preferred that the heating temperature during the dehydrogenationtreatment be 150 to 450° C. If the heating temperature is lower than150° C., the semiconductor film may not be sufficiently crystallized. Ifthe heating temperature exceeds 450° C., a substrate or a semiconductorfilm may be damaged. The heat treatment temperature is furtherpreferably 180° C. to 350° C., with 200° C. to 300° C. beingparticularly preferable.

The heating time is preferably 0.1 to 1200 minutes. If the heating timeis shorter than 0.1 minute, crystallization of the film may beinsufficient since the treating time is too short. A heating timeexceeding 1200 minutes is too long to be productive. A heat treatmenttime of 0.5 minute to 800 minutes is further preferable.

In respect of controlling the hydrogen concentration in thesemiconductor film, the above-mentioned temperature and time conditionsare preferable. If the conditions fall outside the above-mentionedrange, the hydrogen concentration in the semiconductor film may notsatisfy the range stipulated in the invention, which may result in adecrease in mobility of the thin film transistor.

Hydrogenation and crystallization of the semiconductor film may beconducted immediately after the formation of the semiconductor film orafter the formation of other constituting elements such as source/drainelectrodes or the like.

In the invention, since the semiconductor film contains a hydrogenelement, stability of semiconductor properties is improved. Therefore,even if heat treatment conditions during production, in particular,humidity conditions or the like if a heat treatment is conducted in theair, vary, a thin film transistor having stable performance can beproduced.

The production method of the invention is particularly suited as themethod for producing a channel-etch thin film transistor. Since thesemiconductor film of the invention is crystalline, as the method forforming source/drain electrodes and a channel part from a thin film of ametal such as Al, an etching step using photolithography can be used.That is, the semiconductor film is not etched with an etching solutionfor removing a metal thin film, whereby a metal thin film is selectivelyetched. The production method may be a method for producing anetch-stopper thin film transistor.

EXAMPLES Example 1 (A) Fabrication of a Thin Film Transistor

A channel-etch thin film transistor shown in FIG. 3 was fabricated.

A conductive silicon substrate 10 provided with a 200 nm-thick thermallyoxidized film (SiO₂ film) was used. The thermally oxidized film servedas the gate insulating film 30 and the conductive silicon part served asthe gate electrode 20.

Using a target formed of high-purity indium oxide (manufactured byShonan Electron Material Laboratory, an oxide of a metal with an atomicvalency of positive tetravalency or higher; as a representative example,one containing Si, Ti and Sr in a total amount of 0.09 ppm, an oxide ofa metal with an atomic valency of positive divalency or lower: as arepresentative example, one containing Na, K, Mg and Zn in a totalamount of 0.8 ppm), a 40 nm-thick semiconductor film 40 was formed onthe gate insulating film 30 by the sputtering method. Sputtering wasconducted as follows. After conducting vacuum evacuation until the backpressure became 5×10⁻⁴ Pa, the pressure was adjusted to 0.6 Pa byflowing an argon gas containing 8 vol % of hydrogen at 9.0 seem andoxygen at 1.0 seem (that is, the hydrogen concentration in the filmforming atmosphere was 7.2 vol %). Sputtering was conducted at asputtering power of 100 W, with the substrate temperature being 150° C.

Thereafter, in order to dehydrogenate and crystallize the semiconductorfilm, the pressure inside the apparatus was set to 30 Pa with an argongas, and the film was retained at 250° C. for 30 minutes,

After cooling the substrate temperature to room temperature, amolybdenum metal film (200 nm) was formed on the semiconductor film 40.

A resist was applied on the molybdenum metal film, and the resultant wasprebaked at 80° C. for 15 minutes, Thereafter, the resist film wasirradiated with UV light (light intensity: 300 mJ/cm²) through a mask,and developed with a 3 wt % tetramethylammonium hydroxide (TMAH). Afterwashing with pure water, the resist film was post-baked at 130° C. for15 minutes, whereby a resist pattern having a desired shape ofsource/drain electrodes was formed.

By treating the substrate provided with a resist pattern with a mixedacid of phosphoric acid, acetic acid and nitric acid, the molybdenummetal film was etched. After peeling the resist off, and the resultantwas then washed with pure water, dried by air blowing to form the sourceelectrode 50 and the drain electrode 52, whereby a thin film transistor(the distance (L) between the source/drain electrodes of the channelpart 60 was 10 μm and the channel width (W) was 50 μm) was fabricated.

This thin film transistor had normally-off properties with a fieldeffect mobility of 82 cm²/V·sec. an on-off ratio of 10⁸, a thresholdvoltage (Vth) of 0.5V and an S value of 0.7V/dec. The outputcharacteristics showed a clear pinch-off.

(B) Evaluation of a Semiconductor Film

On a quarts glass substrate, a semiconductor film was formed under thesame conditions as in the sputtering in (A), The resulting semiconductorfilm (before dehydrogenation and crystallization) was analyzed by theX-ray diffractometry (XRD), A peak derived from the bixbyite structureof indium oxide was not observed, which means that the film wasamorphous. The hydrogen content of the semiconductor film was measuredand found to be 3.53 at %. The hydrogen content was measured by thehydrogen forward scattering spectrometry method.

Thereafter, the pressure inside the apparatus was set to 30 Pa with anargon gas, and the film was retained at 250° C. for 30 minutes. Theresulting semiconductor film was analyzed by the X-ray diffractometry(XRD), and a peak derived from the bixbyite structure of indium oxidewas observed. The hydrogen content was 3.13 at %.

Example 2

The etch-stopper thin film transistor shown in FIG. 4 was fabricated bythe photoresist method.

In the same manner as in Example 1, on the conductive silicon substrate10 provided with a thermally oxidized film (SiO₂ film), a 40 nm-thicksemiconductor film 40 was formed by the sputtering method by using atarget formed of high-purity indium oxide.

Sputtering was conducted as follows. After conducting vacuum evacuationuntil the back pressure became 5×10⁻⁴ Pa, the pressure was adjusted to0.5 Pa by flowing an argon gas containing 3 vol % of hydrogen at 9.0seem and oxygen at 1.0 sccm. Sputtering was conducted at roomtemperature at a sputtering power of 100 W, with the substratetemperature being room temperature.

Thereafter, by using aluminum oxide as a target, a 10 nm-thick film wasformed by the RF sputtering method. Further, a 190 nm-thick film wasformed thereon by using a silicon oxide target.

A resist was applied on the aluminum oxide-silicon oxide film formed onthe semiconductor film 40, and the resultant was prebaked at 80° C. for15 minutes. Thereafter, the resist film was irradiated with UV light(light intensity: 300 mJ/cm²) through a mask, followed by development in3 wt % tetramethylammonium hydroxide (TMAH). After washing with purewater, the resist film was post-baked at 130° C. for 15 minutes, wherebya resist pattern of an etch stopper in a desired shape was formed.

The substrate provided with a resist pattern was transferred to a dryetching apparatus, and dry etching was conducted with CF₄ gas, Further,the surface was washed and reduced by plasma by using argon containing9% of hydrogen gas. Thereafter, the resist was peeled off, and theresultant was washed with pure water, and dried by air blowing to forman etch stopper 70.

Thereafter, a molybdenum metal film was formed in a thickness of 300 nmon the semiconductor film 40 and the etch stopper 70.

A resist was applied on the molybdenum metal film, and the resultant wasprebaked at 80° C. for 15 minutes. Thereafter, the resist film wasirradiated with UV light (light intensity: 300 mJ/cm²) through a mask,followed by development in 3 wt % tetramethylammonium hydroxide (TMAH),After washing with pure water, the resist film was post-baked at 130° C.for 15 minutes, whereby a resist pattern of source/drain electrodes in adesired shape was formed.

The molybdenum metal film was etched by treating the substrate providedwith a resist pattern with a mixed acid of phosphoric acid, acetic acidand nitric acid. The indium oxide film was simultaneously etched.Thereafter, the resist was peeled off, and the resultant was washed withpure water, and dried by air blowing to form the source electrode 50 andthe drain electrode 52, whereby a thin film transistor (the distance (L)between the source/drain electrodes of the channel part 60 was 10 μm andthe channel width (W) was 50 μm) was fabricated.

Thereafter, in order to dehydrogenate and crystallize the semiconductorfilm, the thin film transistor was subjected to a heat treatment at 300°C. for 30 minutes in the air in a hot air heating furnace.

This thin film transistor had normally-off properties with a fieldeffect mobility of 86 cm²/V-sec, an on-off ratio of 10⁸, a Vth of 0.1Vand an S value of 0.2V/dec. The output characteristics showed a clearpinch-off. A shift in voltage (Vth) after applying to the gate electrodea voltage of 20V for 100 minutes was 0.1V.

(B) Evaluation of Semiconductor Film

On a quarts glass substrate, a semiconductor film was formed under thesame conditions as in the sputtering conditions mentioned above. Theresulting semiconductor film (before dehydrogenation andcrystallization) was analyzed by the X-ray diffractometry (XRD). A peakderived from the bixbyite structure of indium oxide was not observed,which means that the film was amorphous, The hydrogen content of thesemiconductor film was measured and found to be 1.34 at %.

Thereafter, the semiconductor film was heat-treated at 300° C. for 30minutes in air in a hot air heating furnace. The resulting semiconductorfilm was analyzed by the X-ray diffractometry (XRD). A peak derived fromthe bixbyite structure of indium oxide was observed. The hydrogencontent of the semiconductor film was 0.11 at %.

Example 3

A thin film transistor was fabricated in the same manner as in Example2, except that, instead of the target formed of high-purity indiumoxide, an indium oxide target (an oxide of a metal with an atomicvalency of positive tetravalency or higher: as a representative example,one with a total amount of Sn, Ti and Zr<0.1 ppm, an oxide of a metalwith an atomic valency of positive divalency or lower: as arepresentative example, one with a total amount of Na, K, Mg and Zn=1ppm) containing boron oxide, aluminum oxide, gallium oxide, scandiumoxide, yttrium oxide, lanthanum oxide, praseodymium oxide, neodymiumoxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide,dysprosium oxide, holmium oxide, erbium oxide, ytterbium oxide, andlutetium oxide respectively in a total amount of 2 at % was used.

The resulting thin film transistor had normally-off properties, with afield effect mobility of 60 cm²/V·sec or more, an on-off ratio of about10⁸, a Vth of about 0.3V and an S value of 0.5V/dec or less. The outputcharacteristics showed a clear pinch-off. A shift in voltage (Vth) afterapplying to the gate electrode a voltage of 20V for 100 minutes was 0.2Vor less.

Each of the semiconductor films was crystalline and had a hydrogencontent of 1.2 at % to 3.7 at %.

Comparative Example 1

A thin film transistor was fabricated in the same manner as in Example1, except that, as the sputtering target, a target comprising indiumoxide with a purity of 99.9% (an oxide of a metal with an atomic valencyof positive tetravalency or higher: as the representative example, onewith a total amount of Sn, Ti and Zr=200 ppm, an oxide of a metal withan atomic valency of positive divalency or lower: as the representativeexample, one with a total amount of Na, K, Mg and Zn=60 ppm) was used asthe sputtering target, and sputtering was conducted in an atmosphere of100%-purity argon and 100%-purity oxygen, with an oxygen concentrationbeing 10 vol %.

The resulting thin film transistor had normally-on properties, with afield effect mobility of 3.1 cm²/V·sec or more, an on-off ratio of about10⁴, a Vth of −5.1V and an S value of 7.3V/dec or less. The outputcharacteristics showed a clear pinch-off. A shift in voltage (Vth) afterapplying to the gate electrode a voltage of 20V for 100 minutes was 1.4Vor less.

The semiconductor film was crystalline and had a hydrogen content ofless than 0.01 at %.

Comparative Example 2

An attempt was made to fabricate a thin film transistor in the samemanner as in Example 1, except that, as the sputtering target, a targetcomprising an indium oxide, gallium oxide and zinc oxide with a purityof 99.9% (In:Ga:Zn=1:1:1 (atomic ratio)) was used and sputtering wasconducted in an argon gas with a hydrogen content of 1 vol % and100%-purity oxygen, with the hydrogen concentration and the oxygenconcentration being adjusted to 0.86 vol % and 4 vol %, respectively.

However, during the process of etching the molybdenum metal film, thesemiconductor film was dissolved, and hence, a thin film transistorcould not be obtained,

The semiconductor film formed in Comparative Example 2 remainedamorphous after dehydrogenation and crystallization. Therefore, thesemiconductor film was dissolved when the molybdenum metal film wasetched.

INDUSTRIAL APPLICABILITY

The thin film transistor of the invention can be preferably used in adisplay panel, an RFID tag, and a sensor such as an X-ray detectorpanel, a finger print sensor and a photosensor.

The method for producing a thin film transistor of the invention isparticularly suited for the production of a channel-etch thin filmtransistor.

Although only some exemplary embodiments and/or examples of thisinvention have been described in detail above, those skilled in the artwill readily appreciate that many modifications are possible in theexemplary embodiments and/or examples without materially departing fromthe novel teachings and advantages of this invention. Accordingly, allsuch modifications are intended to be included within the scope of thisinvention.

The documents described in the specification are incorporated herein byreference in its entirety.

1. A thin film transistor comprising a gate electrode, a gate-insulatingfilm, an oxide semiconductor film in contact with the gate-insulatingfilm, and source and drain electrodes which connect to the oxidesemiconductor film and are separated with a channel part therebetween,wherein the oxide semiconductor film comprises a crystalline indiumoxide which comprises a hydrogen element, and has the bixbyitestructure. The limitation of the oxide semiconductor film having thebixbyite structure is supported by Examples 1 and 2 ([0052] and [0061].2. The thin film transistor according to claim 1 wherein the oxidesemiconductor film further comprises a positive trivalent metal oxideother than indium oxide.
 3. (canceled)
 4. The thin film transistoraccording to claim 2 wherein the positive trivalent metal oxide otherthan indium oxide is one or more oxides selected from boron oxide,gallium oxide, scandium oxide, yttrium oxide, lanthanum oxide,praseodymium oxide, neodymium oxide, samarium oxide, europium oxide,gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbiumoxide, ytterbium oxide, and lutetium oxide.
 5. A method for producingthe thin film transistor according to claim 1 comprising the steps of,forming a semiconductor film comprising indium oxide which comprises ahydrogen element, patterning the semiconductor film, dehydrogenating andcrystallizing the semiconductor film, and forming source and drainelectrodes such that the electrodes connect to the semiconductor film.6. The method for producing a thin film transistor according claim 5wherein in the step of forming the semiconductor film, the content involume of hydrogen molecules and/or water molecules in the film-formingatmosphere is 1% to 10%.
 7. The method for producing a thin filmtransistor according to claim 5 which is a method for producing an etchstopper thin film transistor.
 8. The method for producing a thin filmtransistor according to claim 5 which is a method for producing achannel etch thin film transistor.
 9. The method for producing a thinfilm transistor according to claim 5 which is a method for producing anetch stopper thin film transistor.
 10. The thin film transistoraccording to claim 2 wherein the positive trivalent metal oxide otherthan indium oxide is one or more oxides selected from boron oxide,aluminum oxide, gallium oxide, scandium oxide and yttrium oxide.
 11. Thethin film transistor according to claim 2 wherein the positive trivalentmetal, oxide other than indium oxide is one or more oxides selected fromboron oxide, aluminum oxide and gallium oxide.
 12. The thin filmtransistor according to claim 1, wherein the oxide semiconductor film,is obtainable by forming a semiconductor film comprising indium oxidewhich comprises a hydrogen element, patterning the semiconductor film,and hydrogenating and crystallizing the semiconductor film.